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June 12, 20143 Architectural Methods for Capacity Expansion Cell Splitting Splitting cell into smaller cells and allow additional channels in the smaller cells Let us consider a cellular architecture with cluster size of 7 When traffic load increases, a smaller cell is introduced such that it has half the area of the larger cells In practice only a single small cell will be introduced such that it is midway between two cocahnnels Let suppose that the radius of the smaller split cell is R/2 Let the transmit power of the BS of the smaller cell be the same as the transmit power of the larger cell

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5 Architectural Methods for Capacity Expansion.. From the smaller cell side, S r is maintained From the larger cochannel cell side: The cochannel reuse ratio is now D L /2R with respect to the smaller cell In order to maintain same level of interference the transmit power of BS in the small cell should be reduced, but this will increase the interference observed by MTs in the smaller cell Another alternative is to divide the channels allocated to larger cells into two parts, some of them can be used by smaller cell and some of them can not be used The channels used by the smaller cell in the larger cell only within radius R/2 from the center of the cell, so that the cochannel reuse ratio will be maintained

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June 12, 20146 Architectural Methods for Capacity Expansion.. This is called overlaid cell concept where larger macrocell coexists with a smaller microcell The disadvantage of this approach is that the capacity of the larger cells is reduced. Also the BS in the larger cells will become more complex and there will need for handoffs between the overlays

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June 12, 20147 Architectural Methods for Capacity Expansion.. Using directional Antennas for Cell Sectoring The simplest and most popular scheme for expanding the capacity of cellular systems Reduces the interference and cluster size and increasing capacity The radio propagation is focused in only one direction where it is required, which reduces the interference

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June 12, 20148 Architectural Methods for Capacity Expansion.. The coverage of the cell BS antenna is restricted to part of a cell called a sector by making the antenna directional Cell site locations remain unchanged, and only the antenna used in the site will be changed Increases S r, which enables using lower frequency reuse factor Lower frequency reuse factor allows a larger number of channels per cell, which increases overall capacity

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June 12, 20149 Architectural Methods for Capacity Expansion.. Where J s is the number of interfering cell sites The most popular directional antenna employed in cellular systems are 120 o directional antennas In some cases 60o directional antennas are employed

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June 12, 201414 It is possible to use a cluster size of 4 or 3 with 6-sector cells, because S r will be 21.58 dB or 19.1 dB respectively, which has sufficient margin for AMPS

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June 12, 201415 Architectural Methods for Capacity Expansion.. Disadvantages of Using Sectors Each sector is a new cell with different shape Channels are partitioned between different sectors More handoffs

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June 12, 201416 Architectural Methods for Capacity Expansion.. Lee s Microcell Method One BS per cell with three zone-sites located at the corners of the cell Directional antennas that span 135 o are employed at these zone-sites All three zone-sites act as receivers for signals transmitted by an MT The BS determines which of the zone-sites has the best reception from the MT and uses that zone-site to transmit the signal on the downlink The zone-sites are connected to the BS by high speed fiber links to avoid congestion and delay

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June 12, 201417 Architectural Methods for Capacity Expansion.. Lee s Microcell Method.. Only a single zone site is active at a time to reduce the cochannel interference Cluster size is reduced to three and a capacity of gain of 2.33 is obtained over seven-cell cluster scheme

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June 12, 201419 Architectural Methods for Capacity Expansion Read example 5.12 page 246 in the handouts Using Overlaid Cells Channels are divided among a larger macrocell that coexists with microcell contained entirely in the macrocell BS serves both the maco- microcells R 1, D 1, R 2, and D 2 D 2 /R 2 is larger than D 1 /R 1 so S r for the microcell will be grater than that of the macocell The microcell is called overlay cell and macrocell is call underlay cell Two methods to exploit this situation to increase the capacity

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June 12, 201420 Architectural Methods for Capacity Expansion.. Split-Band Analog Systems BW efficient modulation is used within overlay cells, which is FM S/N is inversely proportional to BW 2 If BW is reduced to half the original value, S/N will be increased four times (by 6dB) If we arrange R 2 and D 2 to have a cochannel reuse ratio that is four times larger than usual, we end up with S r that remains unchanged The overlay cell then can use the FM with half BW of the underlay cell doubling the capacity within the overlay

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June 12, 201424 Architectural Methods for Capacity Expansion.. Reuse Partitioning Channels are divided among a macrocell and a microcell contained entirely in the macrocell BW in both cells remain the same S r for the overlay cell is larger than that of the underlay cell Channels allocated to the microcell may be used in every 3 rd or 4 th microcell Channel allocated to the marcocell may be used in every 7 th or 12 th macrocell This requires additional complexity at the BS and handoffs when an MT moves from microcell to macrocell

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June 12, 201425 Example Reuse partitioning of 7 and 3 If we have AMPS network operating on the infrastructure in the next figure, the required S r is 18 dB. D 1 /R 1 = D 2 /R 2 = 4.6 (EQ. 5.2 in the handouts) Since R2 < R1 D2 < D1 improvement in cochannel reuse ratio because the microcells are not contiguous